Thermochemical cycles

Two of these cycles have an electrolysis step. Although one of the purposes of the thermochemical cycles is to avoid electrolysis and the associated iaefftciencies of electricity production, the electrolysis steps proposed use much less electrical energy than water electrolysis. The Mark 13 is regarded as the most advanced thermochemical cycle, with overall efficiency of about 40%, including the electrolysis step (164). 

Cl-I-Br] the I-Cl distance is greater than the I-Br distance, and in [Br-I-I] I-Br is greater than I-I. On dissociation, the polyhalide yields the solid monohalide corresponding to the smaller of the halogens present, e.g. CsIClj gives CsCl and ICl rather than Csl + Clj. Likewise for CsIBrCl the favoured products are CsCl(s) + IBr(g) rather than CsBr(s) + ICl(g) or Csl(s) + BrCl(g). Thermochemical cycles have been developed to interpret these results. 

When the multiplicity of a complex is the same for ionic or ion-dipole bonds and for covalent bonds, the decision as to which extreme bond type is the more closely approached in any actual case must be made with the aid of less straightforward arguments. Sometimes theoretical energy diagrams can be constructed with sufficient accuracy to decide the question. A discussion of crystals based on the Born-Haber thermochemical cycle has been given by Rabinowitsch and Thilo3), and more accurate but less extensive studies have been made by Sherman and Mayer4). 

Thermochemical properties of reactive molecules can be either measured directly, or obtained indirectly through thermochemical cycles. The thermochemical relationships between standard thermochemical properties are illustrated in Figure 5.2. From these, many different relationships can be found. 

Reaction (41) has indeed been confirmed by pressure measurements and mass spectroscopic analyses (73, 79). The experimental heats for reaction (41) may be introduced in the following thermochemical cycle ... 

Thermochemical Cycles Testing the Formation of Gaseous (Cycle 1) or Adsorbed (Cycle 2) Carbon Dioxide by the Interaction of Carbon Monoxide with Oxygen Preadsorbed on Gallium-Doped Nickel Oxide ... 

Before analyzing the results of these, or similar, thermochemical cycles, the assumptions which have been made must be critically examined. Since the cycles are tested for different surface coverages, it is assumed first that the Q-0 curves represent correctly, in all cases, the distribution of reactive sites—the energy spectrum—on the surface of the adsorbent. This point has been discussed in the preceding section (Section VII.A). It is assumed moreover that, for instance, the first doses of carbon monoxide (8 = 0) interact with oxygen species adsorbed on the most reactive surface sites (0 = 0). This assumption, which is certainly not acceptable in all cases, ought to be verified directly. This may be achieved in separate experiments by adsorbing limited amounts of the different reactants in the same se-... 

Thermochemical Cycle Testing the Formation of Gaseous Carbon Dioxide at the End of the Adsorption Sequence (CO—O2—CO)"... 

One of the conclusions deduced from the thermochemical cycle 2 in Table V, for instance, is that in the course of the catalytic combustion of carbon monoxide at 30°C, the most reactive surface sites of gallium-doped nickel oxide are inhibited by the reaction product, carbon dioxide. This conclusion ought to be verified directly by the calorimetric study of the reaction. Small doses of the stoichiometric reaction mixture (CO + IO2) were therefore introduced successively in the calorimetric cell of a Calvet microcalorimeter containing a freshly prepared sample of gallium-doped... 

Moreover, the use of heat-flow calorimetry in heterogeneous catalysis research is not limited to the measurement of differential heats of adsorption. Surface interactions between adsorbed species or between gases and adsorbed species, similar to the interactions which either constitute some of the steps of the reaction mechanisms or produce, during the catalytic reaction, the inhibition of the catalyst, may also be studied by this experimental technique. The calorimetric results, compared to thermodynamic data in thermochemical cycles, yield, in the favorable cases, useful information concerning the most probable reaction mechanisms or the fraction of the energy spectrum of surface sites which is really active during the catalytic reaction. Some of the conclusions of these investigations may be controlled directly by the calorimetric studies of the catalytic reaction itself. 

Pickard, P., Sulfur-iodine thermochemical cycle, 2006 Annual Merit Review Proc., Hydrogen Production and Delivery, D. Nuclear Energy Initiative, http //www.hydrogen.energy.gov/ annual review06 delivery.html. 

The enthalpy of formation of a compound is a so-called thermodynamic state function, which means that the value depends only on the initial and final states of the system. When the formation of crystalline NaCl from the elements is considered, it is possible to consider the process as if it occurred in a series of steps that can be summarized in a thermochemical cycle known as a Born-Haber cycle. In this cycle, the overall heat change is the same regardless of the pathway that is followed between the initial and final states. Although the rate of a reaction depends on the pathway, the enthalpy change is a function of initial and final states only, not the pathway between them. The Born-Haber cycle for the formation of sodium chloride is shown as follows ... 

From the standpoint of energy, the processes of separating the crystal lattice and solvating the ions can be related by means of a thermochemical cycle of the Born-Haber type. For an ionic compound MX, the cycle can be shown as follows ... 

By means of appropriate thermochemical cycles, it is possible to calculate proton affinities for species for which experimental values are not available. For example, using the procedure illustrated by the two foregoing examples, the proton affinities ofions such as HC03-(g) (1318 k J mol-1) and C032-(g) (2261 kj mol-1) have been evaluated. Studies of this type show that lattice energies are important in determining other chemical data and that the Kapustinskii equation is a very useful tool. 

In Chapter 1 we discussed the electron affinities of atoms and how they vary with position in the periodic table. It was also mentioned that no atom accepts two electrons with a release of energy. As a result, the only value available for the energy associated with adding a second electron to O- is one calculated by some means. One way in which the energy for this process can be estimated is by making use of a thermochemical cycle such as the one that follows, showing the steps that could lead to the formation of MgO. 

The removal of two electrons from a magnesium atom is highly endothermic, as is the addition of two electrons to an oxygen atom. In spite of this, MgO forms readily from the elements. Write a thermochemical cycle for the formation of MgO and explain the process from the standpoint of the energies involved. 

However, the adduct is not stable enough to exist in significant amounts at a temperature of 116 °C, the boiling point of pyridine. As a result, such interactions are studied in solutions, although it is the strength of the bond that would be produced in the gas phase that is desired. A thermochemical cycle... 

According to the thermochemical cycle in Scheme IV, the activation free energy for electron transfer AG in the encounter complex is given by (16) ... 

DuBois et al. carried out extensive studies on the thermodynamic hydricity of a series of metal hydrides [13, 15-19]. The determination of thermodynamic hydricity generally requires several measurements (coupled with known thermochemical data) to constitute a complete thermochemical cycle. As with other thermodynamic cycles, obtaining reliable values in an appropriate solvent can be a difficult challenge, and this is sometimes coupled with problems in obtaining reversible electrochemical data. Scheme 7.2 illustrates an example in which the hydricity of cationic monohydrides have been determined. 

Eq. (8) requires determination of the two-electron oxidation potential of L M by electrochemical methods. When combined with the two-electron reduction of protons in Eq. (9), the sum provides Eq. (10), the AGh- values of which can be compared for a series of metal hydrides. Another way to determine the AGh-entails the thermochemical cycle is shown in Scheme 7.3. This method requires measurement of the K of Eq. (11) for a metal complex capable of heterolytic cleavage of H2, using a base (B), where the pK., of BH+ must be known in the solvent in which the other measurements are conducted. In several cases, Du-Bois et al. were able to demonstrate that the two methods gave the same results. The thermodynamic hydricity data (AGh- in CH3CN) for a series of metal hydrides are listed in Table 7.4. Transition metal hydrides exhibit a remarkably large range of thermodynamic hydricity, spanning some 30 kcal mol-1. 

High temperature nuclear thermochemical cycles, hydrogen production by, 13 847-849... 

Thermochemical cycles based on solar energy are another long-term option for hydrogen production in countries with favourable climatic conditions. 

Figure 2.1 Thermochemical cycle, showing how to relate the enthalpy of the experimental reaction 2.1 with reaction 2.2, where reactants and products are in their standard states.

Figure 2.2 Thermochemical cycle relating the enthalpies of reaction 2.2 at 298.15 Kand 310 K.

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